The present state of the art in silicon photovoltaic solar cells is far from encouraging on the basis of electrical power output per unit cell cost. Present costs run at about $25 per watt in quantity, and about $40 per watt in single packaged assemblies, which is prohibitably high for any commercial or household direct power conversion installation.
It is quite unlikely that the future cost of the silicon photovoltaic solar cell can be substantially reduced because of the basic material processing and fabrication techniques involved. Only a very large and steady market demand can eventually lower the costs appreciably, which will be impossible to achieve, quickly from the present very high cost levels.
Some progress is now being made in the development of direct solar conversion systems using conventional silicon solar cells receiving concentrated solar rays to increase the photo light intensity and thereby the power output, but this arrangement using circular parabolic reflectors is made complex by a liquid cooling adaptation requirement and the bulkiness of the equipment.
It appears that this approach to increasing the electrical power output of each solar cell is practical and has cost reduction merit, provided that the light heat level received by the solar cell is kept equal to, or slightly below, the optimum exposure levels of the specific solar cell, so that the cell life span is not adversely affected, and the direct conversion geometry and means kept as simple and inexpensive as possible.
It is believed that the presently advocated direct solar conversion arrangement, using linear parabolic reflectors (L.P.R's) will ideally meet the functional requirements as previously described. The linear parabolic reflectors (L.P.R.) is considerably less expensive to produce than a corresponding circular parabolic reflector because of the simple linear, uniform geometry involved, and simplified production forms and techniques.
The circular dish parabolic reflector produces a very hot "spot", concentration area for a relatively large collection exposed surface area, while the linear parabolic reflector produces a moderately hot "strip" concentration area for a relatively smaller collection surface area.
The circular parabolic reflector arrangement has the advantage of requiring a minimum number of solar cells for a system which will require a very large total exposure surface area for nearly full house-power requirements, while the linear parabolic ic reflector configuration has a decided advantage for a dual, direct electric and indirect hot-water steam solar conversion system. The moderately hot 250 degrees to 300 degrees F. temperature level "strip" of the linear parabolic reflector is ideal for the dual solar conversion means, where both a continuous line of solar cells and parallel, "piggy-back" flash boiler pipe can be uniformly exposed to an approximate 16:1 concentration of solar rays.
A direct solar conversion arrangement using the circular type of parabolic reflector has a cost projection which is still prohibitively high for a wide market acceptance, in the ($5000 to $7000) price range for approximately half the average house power requirements (6 KW/hr.) It is projected that this present arrangement can produce equivalent KW output performance at a present cost of between ($3500-$4500) for a one-family house, or full average housepower (12 KW/hr), at an equivalent present cost to the prior solar power system.
Most of the basic water heating components of the solar power system are not new or novel, and are now in use in several solar water heating and direct conversion installations, with the exception of the unique high concentration type of linear parabolic reflectors (L.P.R.'s), which are a new innovation in the art.
Linear parabolic reflectors (L.P.R.'s) have been used in the past with a concentration ratio of about 5:1, in successful water-pumping irrigation projects, in which the heated water-to-steam produced drove a large steam engine water pump. The Nile River water pumping irrigation system utilized relatively large diameter pipe and L.P.R.'s to heat the river water to steam sufficient to drive a 50 hp steam engine.
Because of the current necessity of limiting the exposure area for house power applications, the circulating water must be preheated in several conventional water heating units to about 135 degrees F. before entrance into the multiple, parallel flash boiler pips. For a practical steam horsepower rating of about 10-15 hp, it is desirable to use multiple small diameter pipes rather than a single large pipe size for the flash boiler pipe installation, since both the L.P.R.'s and solar exposure area can be adapted to match the roof area of the average one-family house. Excessively large L.P.R.'s are not desirable, since they are vunerable to wind and storm damage and subsequent repair and servicing.
Conventional shallow "dish" cross-section L.P.R.'s reflect and concentrate solar rays at the central linear apex or focal line, and are therefore limited in the amount of solar ray concentration that can be realized. In addition, the generally short height cross-section of the conventional L.P.R.'s is not structurally rigid over long distances (poor section modulus), making it necessary to provide a substantial backing for structural reinforcement, for sun-following oscillation installations.
The present high concentration type of L.P.R. cross-section configuration is aimed at improving both of these features, the concentration ratio and section modulus, so that optimum concentration and structural stiffness will be achieved.
These desirable results are gained by adding extra concave reflector sections to the basic lower parabolic cross-section, above the focal line of the basic parabolic reflector section. Another design approach is to adopt a bi-level reflector geometry, in which lower, side concave reflectors transmit solar rays to lower, central mirrors, and then upward to the line of solar cells.
To be practical and effective, the solar panel system must have an accurate sun-following linkage arrangement connecting each of the pivoted L.P.R.'s, so that the solar rays are kept closely parallel to the L.P.R.'s vertical centerline for uniform solar reflective concentration during each sunlit day.
The L.P.R. panel length should be kept as short as possible up to a maximum of about twenty feet to provide convenient and accurate linkage displacement control, and to minimize the counter affects of wind gusts and non-uniformly loading on the L.P.R. panels. Mirrored surfaces are a must for the L.P.R. panels.
It will be necessary to keep the L.P.R. panels clean and clear of leaves, twigs, and debris at all times, so that the highly reflective surfaces are not obstructed. The accumulation of dust, soot dirt and oil films on the reflective surfaces L.P.R. panels will substantially cut down on the light and heat intensity received by the solar cells and flash boiler pipe at the central focal zone.